A resistance-switching element is a circuit element whose electrical resistance is determined by its previous current-voltage history in such a way that an applied voltage of sufficient magnitude can switch it between a high-conductance state and a low-conductance state. The terms “memristor” or “resistive random access memory” (ReRAM or RRAM) is often used to refer to a passive two-terminal resistance-switching element fabricated with a thin insulating film sandwiched between two conductors.
Memristors are among the most promising emerging memory devices to replace flash, DRAM, and possibly SRAM. The desirable analog behavior of memristors may also make them a candidate for neuromorphic circuit applications. Currently known types of memristors are believed to work because large-signal voltages (typically >1 V) across the device may be used to shift the spatial distribution of ions in the insulator, which in turn acts to change the small-signal (typically <1 V) resistance of these devices.
The insulating materials used until now in memristors are generally transition metal oxides (TMO) (e.g., as films) containing mobile oxygen ions and oxygen vacancies, such as TiO2, Ta2O5, WO3, HfO2, NiO, and Nb2O5. Electrodes are typically made from electrically conductive metals. In many instances, a pair of metals is favored for the respective electrodes, in which one metal more readily reduces the insulator material than the other.
Control of mobile oxygen ions and oxygen vacancies is key for providing a stable memristor. Switching between states in a memristor involves the formation of oxygen vacancies in the TMO film by oxygen transfer into or from the active metal electrode. This oxygen transfer step requires minimal energy. For this reason, resistive memory devices exhibit beneficial, low power consumption for read/write operations. However, the lack of an energy barrier also gives rise to the very low reliability (e.g., low endurance and state retention) observed in these devices so far. It may also give rise to some forms of random noise in the reading of the device state. Thus, there is a need for additional methods and components to control these mobile species in order to increase reliability, state retention, endurance, and/or reduce noise in memristive devices.